1. Field of the Invention
The present invention relates in general to a modified color reduction system for four color printing which depends on the tonal value where the color components of the chromatic printing inks yellow (Y), magenta (M) and cyan (C) are reduced and the amount of black printing ink (K) is increased so as to compensate for the decrease in the chromatic printing inks.
2. Prior Art
The article entitled "FOGRA-Forschungsbericht 1.203, Untersuchungen zur Veranderung des Farbsatzaufbaus durch geanderte Farbauszuge fur Teilfarbe Schwarz" by Gunter Otschik, page 4, right-hand column, Chapter 3.1, discloses how it is possible to work with total chromatic synthesis of printing in color reduction in four color printing with an intensive black, in other words, an intensive but expensive printing ink "black". Total achromatic synthesis of printing inks means in reproduction technology that printing is accomplished with "black" in the reproduction of gray tone values and in the extremely dark tones or tints and, in other words, there are no chromatic color components contained and printed in the color separations yellow, magenta and cyan at these image gray tone values and in the extremely dark tones.
Normally cheaper printing inks for the color black are utilized and this results, however, in a gray cast occurring when 100% black is printed. For this reason, the color separations of the chromatic printing inks are not reduced to zero at these tonal values but rather a residue of chromatic color which gives rise to a higher density is printed over the entire tonal tint range in gray tones up to the deepest black (shadow) with the residual values decreasing progressively toward lighter tones and tints.
Thus, there is a danger that color casts will occur in the reproduction of gray tones over the entire tonal or tint range when fluctuations occur in the equalibrium of the chromatic printing inks Y, M and C, and this can easily occur during the printing process as is pointed out in the previously referenced citation FOGRA-Forschungsbericht, pages 3 and 4, Chapters 1 and 2.
Another disadvantage as compared to complete color reduction is that in case the gray tones are reproduced by chromatic components, a higher ink consumption of the chromatic inks occurs then when one works and uses an intense black ink.
So as to achieve a reliable gray balance using a small amount of ink for the chromatic components, a color reduction depending on the tonal values has previously been proposed for four color printing and this is described in the European patent application No. 82103090.5 which corresponds to the copending U.S. patent application Ser. No. 479,733. In this patent application, the color components of the chromatic printing inks yellow (Y), magenta (M) and cyan (C) are reduced and the amount of the black printing ink (K) is increased to compensate for this reduction and a nearly total achromatic synthesis of printing is used up to a gray tone value which corresponds to the full tone or tint of the black printing ink employed and the color components of the chromatic printing inks are then increased from this gray tone value toward the darkest shadow of the image.
Very good results can be obtained with this method but it turns out that there are instances in which so called breaks or steps occur in the light tonal range of the image. An example of such breaks occur in the reproduction of pastel shades or tones as, for example, in the magenta range when a transition to a gray tone value occurs in such color area. When total achromatic synthesis of printing is used, magenta, but less magenta, is printed in the magenta-colored area, but no magenta is present in the gray area so that formation of a contour line or a jump of the tonal value, which is not present in the original image, occurs. This results in a disruptive effect when the reproduction is viewed by an observer.